This disclosure relates to a voltage control circuit for generating and outputting a constant DC voltage from an input DC voltage.
FIGS. 2(a) and 2(b) are block diagrams of conventional voltage control circuits. In FIG. 2(a), the voltage control circuit includes an NPN-type transistor (hereinafter referred to as “NPN”) 23 having a collector connected to an input terminal 21 and an emitter connected to a output terminal 22. A resistor 24 is connected between the collector and a base of the NPN 23, as is described in Japanese Patent Application Laid Open Publication No. 2006-127093, which is incorporated by reference. The base of the NPN 23 is connected to the ground voltage GND through the NPN 25 and Zener diode 26, which are serially connected. In addition, the base of the NPN 25 is connected to an output terminal 22, and the emitter of the NPN 25 is connected to the output terminal 22 through a resistor 27.
In the voltage control circuit of FIG. 2(a), when an input voltage VI is applied to the input terminal 21, a current starts to flow through the resistor 24 to turn on the NPN 23, causing an output voltage VO at the output terminal 22. As a result, a Zener current flows through the Zener diode 26 through the resistor 27. Since the base-emitter voltage (VBE) of the NPN 25 is approximately 0.6V of constant voltage, a current flowing through the resistor 27 becomes a constant value corresponding to the resistance of the resistor 27. Consequently, the emitter voltage of the NPN 25 becomes the Zener voltage arising at the Zener diode 26 by the constant Zener current. Therefore, the output voltage VO becomes a sum voltage of the Zener voltage of the Zener diode 26 and the base remitter voltage VBE of the NPN 25, and then a constant output voltage VO can be obtained independently from the value of a load connected to the output terminal 22.
Additionally, in FIG. 2(b), the voltage control circuit includes a PNP-type transistor (hereinafter referred to as “PNP”) 33 having an emitter connected to the input terminal 31, a collector connected to an output terminal 32, and a base connected to a collector of a NPN 35 through a resistor 34, as is described in Japanese Patent Application Laid Open Publication No. H5-250048, which is incorporated by reference. The emitter of the NPN 35 is connected to a current limiter 37 through Zener diode 36. A voltage divider, including resistors 38, 39, is connected between the output terminal 32 and the ground voltage GND, and the output voltage VO is divided by the voltage divider to be provided to an error amplifier 40. The error amplifier 40 outputs a voltage corresponding to the differential between the divided voltage of the output voltage VO and a reference voltage REF, and the voltage is provided to the base of the NPN 35 through a resistor 41.
In the voltage control circuit of FIG. 2(b), the output voltage VO divided by the voltage divider and the reference voltage REF are compared with each other by the error amplifier 40, and the collector current of the NPN 35, used as a driving current, is controlled based on the result of the comparison. The collector current of the NPN 35 controls the base current of the PNP 33, which is used for controlling the voltage, so that the output voltage VO becomes proportional to the reference voltage REF. Consequently, the output voltage VO can be maintained at a constant voltage while the load connected to the output terminal 32 is varied and while the input voltage VI is varied.
Japanese Patent Application Laid Open Publication No. 2006-202146, which is incorporated by reference, also provides background to the present disclosure.
In the voltage control circuit of FIG. 2(a), the Zener current flowing through the Zener diode 26 is not only the current flowing through the resistor 27 from the output terminal 22, but also includes the current flowing through the resistor 24 and the NPN 25 from the input terminal 21. Consequently, in the case where the input voltage VI is constant, the Zener current becomes approximately constant and a stable output voltage VO can be obtained; however, in the case where the input voltage VI varies, the Zener current varies and the Zener voltage varies. Accordingly, the output voltage VO is influenced by variation of the input voltage VI.
In the voltage control circuit of FIG. 2(b), the stable output voltage VO can be obtained independently from the variations of the input voltage VI and variations of the load current; however, since the error amplifier 40 and a circuit for generating the reference voltage REF are necessary, a larger circuit may be required. In addition, since the supply voltage of the error amplifier 40 is provided from the input voltage VI, when a high input voltage VI (for example, 24V) is applied, an error amplifier 40 having a high voltage rating becomes necessary.